1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an image forming process such as an electrophotographic process or an electrostatic recording process, and a heat fixing apparatus for heating fixing, in an image forming process unit of such image forming apparatus, an unfixed toner image formed and borne on a recording material (a transfer material, a printing sheet, a photosensitive sheet or an electrostatic recording sheet).
2. Related Background Art
As a fixing apparatus equipped in an image forming apparatus employing an electrophotographic process or an electrostatic recording process, there is widely employed so-called heat fixing apparatus in which a recording material bearing an unfixed toner image is passed through a nip portion (fixing nip portion) formed by a fixing roller and a pressure roller which are rotated in mutual contact whereby a permanent image is fixed on the recording material.
An example of a conventional heat fixing apparatus is shown in FIG. 12A. Referring to FIG. 12A, a fixing roller 40 constituting heating means is provided with a hollow aluminum core 42 of a thickness of 0.5 to 4 mm for satisfying a mechanical strength, in which a halogen lamp 41 is provided for effecting heating, by a power supply from an unrepresented power source, sufficient for fusing the toner on the recording material from the interior of the hollow core 42.
Also for fixing the toner, on the recording material, thereto without offsetting, a releasing layer 43 of an excellent releasing property such as of polytetrafluoroethylene (PTFE) or perfluoroalkoxy-tetrafluoroethylene copolymer (PFA) is provided on the outside of the hollow core 42. The releasing layer 43 is formed in a tube shape or formed by electrostatic spraying or dip coating.
Also in order to prevent an offset phenomenon generated by a charging of the surface of the fixing roller by the conveying of the recording material, the releasing layer 43 may contain a conductive material such as carbon black. Also the hollow core 42 of the fixing roller 40 is either electrically grounded, or grounded through a diode, or is given a bias by an unrepresented bias applying means, thereby preventing a charging of the surface of the fixing roller leading to generation of an offset image.
Also a thermistor 44 is maintained in contact with the surface of the fixing roller 40 for detecting the surface temperature of the fixing roller, and on/off controls the power supply to the halogen lamp 41 for heating the toner image on the recording material at an appropriate temperature.
On the other hand, a pressure roller 50 is pressed to the fixing roller 40 by unrepresented pressure springs on both ends of a longitudinal direction of the rollers, for pinching and conveying the recording material. The pressure roller 50 is formed, on a metal core 51, by forming an elastic layer molded with silicone rubber or a sponge elastic layer 52 constituted of foamed silicone rubber, and thereon a releasing layer 53 such as of PTFE, PFA or FEP similar to that in the fixing roller, formed as a tube or by coating.
Thus, by the elasticity of the pressure roller 50, a sufficient nip width can be formed between the rollers. A toner image on a recording material P pinched and conveyed in the nip portion can be fixed by a heating from the fixing roller 40.
Japanese Patent Applications Laid-open Nos. 63-313182, 2-157878, 4-44075 and 4-204980 disclose a method of not supplying an electric power to the heat fixing apparatus in a standby state thereby suppressing the electric power consumption, more specifically a heat fixing method by a film heating process in which the toner image on the recording material is fixed across a thin film passed between the heater unit and the pressure roller.
FIG. 12B shows a schematic configuration of a film heating method. More specifically, referring to FIG. 12B, heating means 60 is constituted of a heating member (hereinafter represented as “heater”) 61 fixed on a heat-resistant stay holder (support member) 62, and a thin film (hereinafter represented as “fixing film”) resistant to the heat of the heater 61, and, in order to form a nip portion (fixing nip portion) of a predetermined nip width, a predetermined pressure is given to the elastic pressure roller 50 by unrepresented pressurizing means.
The heater 61 is constituted by forming, on a ceramic substrate such as of alumina, a heat-generating resistance layer and a protective layer such as a glass layer or a polyimide layer, and is heated by a current supply and controlled at a predetermined temperature by temperature detecting means 64 provided on a rear surface of the heater 61. A fixing film 63 is a cylindrical or endless belt, or a rolled web member, which is conveyed in a direction indicated by an arrow in sliding contact with the surface of the heater 61 at the fixing nip portion, by unrepresented drive means or by a rotating force of the pressure roller 50.
In a state where the heater 61 is heated and controlled at the predetermined temperature and the fixing film 63 is conveyed in the direction indicated by the arrow, a recording material constituting a member to be heated and bearing an unfixed toner image is introduced between the fixing film 63 and the pressure roller 50 at the fixing nip portion, whereupon the recording material is conveyed through the fixing nip portion in contact with and together with the fixing film 63. In the fixing nip portion, the recording material and the toner image are heated by the heater 61 through the fixing film 63 whereby the toner image on the recording material is heated and fixed. After passing the fixing nip portion, the recording material is separated from the surface of the fixing film 63 and is conveyed.
The fixing film 63 is made as considerably thin as 20 to 70 μm, in order to supply the recording material, constituting the member to be heated, with the heat from the heater 61 in the fixing nip portion. The fixing film 63 has, as shown in FIG. 13A, a three-layered structure of a film base layer 63a, a conductive primer layer 63b and a releasing layer 63c, in which the film base layer 63a is positioned at the side of the heater and the releasing layer 63c is positioned at the side of the pressure roller 50.
The film base layer 63a is constituted of an insulating resinous film such as of polyimide, polyamidimide or PEEK, or a thin metal film such as of SUS or Ni, and is formed with a thickness of about 15 to 60 μm having a heat resistance, a high elasticity and a flexiblity.
Also the film base layer 63a ensures a mechanical strength, such as a tear strength, of the entire fixing film 63.
The conductive primer layer 63b is formed by a thin layer of a thickness of 2 to 6 μm, and is electrically grounded or connected with a diode or a bias applying means in order to prevent charging of the entire fixing film.
The releasing layer 63c is a layer for preventing toner offset to the fixing film 63, and is formed by coating a fluorinated resin of satisfactory releasing property such as PFA, PTFE or FEP with a thickness of about 5 to 10 μm. Also as in the fixing roller, in order to reduce the charging on the surface of the fixing film 63 thereby preventing an electrostatic offsetting, the releasing layer contains a conductive material such as carbon black of a specific resistivity of about 103 to 106 Ωcm.
Also the pressure member 50 has a configuration similar to that of the pressure roller of the heat fixing apparatus of the aforementioned fixing roller type.
In the heat fixing apparatus of the above-described film heating type, the heater is not powered during a standby state but is powered and rapidly heated to a fixable temperature during a period from a reception of a print signal by the image forming apparatus to the arrival of a recording material to the fixing nip portion, whereby the unfixed toner image on the recording material can be heat fixed and a power saving can be attained.
However, the recording material has recently been used in various types, and has diversified in thickness, surface property, resistance etc. For this reason, various drawbacks are encountered in the heat fixing step in the heat fixing apparatus of the image forming process, and have been avoided by various configurations.
For example, in the heat fixing apparatus of the aforementioned prior configuration, at the entry of the recording material into the fixing nip portion, there may result a phenomenon that the unfixed toner image on the recording material scatters in a direction opposite to the conveying direction of the recording material (hereinafter represented as “fixed image tailing”). A mechanism of generation of such fixed image tailing will be explained with reference to FIG. 13B.
As shown in FIG. 13B, the fixed image tailing is generated by a fact that moisture contained in the recording material P is vaporized by a rapid heating in the fixing nip portion and the unfixed toner image T on the recording material P prior to entering the fixing nip is blown off by thus generated vapor 80 in a direction opposite to the conveying direction of the recording material to generate an image defect, which is apt to be generated particularly in a high humidity environment, when the recording material P has a high moisture content and when the image pattern is a lateral line image with a larger line width and a larger toner amount in the unfixed toner image T.
It is also known that such fixed image tailing aggravates in case the vapor generation 80 from the recording material becomes stronger with an increase in the speed of the image forming apparatus.
In the following, there will be shown a configuration for alleviating the fixed image tailing. As shown in FIGS. 12A and 12B, a discharge rubber roller 71 and a discharge roller 72 are provided in a pair, at a downstream side of the fixing nip portion formed by the heating means 40 or 60 and the pressure roller 50, and serve to convey the recording material discharged from the fixing nip portion. The discharge rubber roller 71 is formed by a conductive rubber member and is in an electrically grounded state. Otherwise a brush-shaped grounded conductive member is positioned at the downstream side of the fixing nip so as to be contacted with the recording material during conveying.
Also in the prior configuration explained in the foregoing, the hollow core 42 of the fixing roller 40 and the conductive primer layer 63b of the fixing film 63 are given a bias of a polarity same as that of the unfixed toner image by unrepresented bias applying means. Thus, as the recording material passes the fixing nip portion and contacts the conductive discharge rubber roller 71, a current path is formed through the recording material P to generate a voltage drop between the fixing roller 40 or the fixing film 63 and the recording material P, and an electric field thus generated enhances a supporting power for the unfixed toner image on the recording material, thereby preventing the fixed image tailing.
However, in case the speed of the image forming apparatus easily causing such fixed image tailing is increased, it is necessary to increase the voltage drop between the fixing roller 40 or the fixing film 63 and the recording material P in order to prevent the fixed image tailing, so that it is necessary to increase the bias applied to the hollow core 42 of the fixing roller 40 or the conductive primer layer 63b of the fixing film 63 thereby generating a larger current in the current path formed through the recording material.
However, in the above-explained configuration of forming a current path between the hollow core 42 of the fixing roller 40 or the conductive primer layer 63b of the fixing film 63 and the conductive discharge rubber roller 71 through the recording material, an eventual excessive current in the current path causes a charge injection, to the toner immediately after the fixing nip, of a polarity opposite to the charging polarity, whereby the toner is inverted in the polarity and becomes easily sticking to the fixing roller 40 or the fixing film 63, thereby causing a toner contamination.
In a low-cost heat fixing apparatus not equipped with cleaning means for the surface of the fixing roller 40 or the fixing film 63, the toner contamination is gradually accumulated on the fixing roller 40 or the fixing film 63, or on the pressure roller 50 maintained in contact therewith through heat fixation of a large amount of the recording materials, and thus accumulated toner may occasionally deposited onto the recording material (hereinafter called “blobs”) thereby resulting in an image defect.
Also in case a large potential difference is constantly formed between a conductive part of the fixing roller 40 or the fixing film 63 and a conductive part of the pressure roller 50, substances conveyed by the recording material to the fixing nip, such as paper powder, dusts, fibers from the recording paper and powder scraped off from the photosensitive drum of the image forming unit, may be deposited by an electrostatic force to the surface of the fixing member or the pressure member. Such phenomenon deteriorates the original releasing property, thus enhancing the offset phenomenon or the toner contamination.